Room-Temperature Catalytic Treatment of High-Salinity Produced Water at Neutral pH

Produced waters from hydraulic fracturing (HFPW) operations greatly challenge traditional water treatment technologies due to the high concentrations of total dissolved solids (TDS), highly complex and variable water matrices, and significant residual hydrocarbon content. We recently reported the unusual ability of a PdAu catalyst to degrade phenol in simulated HFPW at room temperature by generating H2O2 in situ from formic acid and air. Phenol removal occurred at TDS levels as high as ∼10 »000 ppm (ionic strength I = 0.3 M), but the catalytic reaction required pH < 4 to proceed. Here, we find that PdAu, Pd, and Au degraded phenol in the pH 5-8 range by using hydroxylamine as the hydrogen source in place of formic acid. Pd exhibited the highest activity, and Au the least. Activity of the monometallic catalysts decreased >70% as TDS increased from 0 to ∼100 »000 ppm (I = 3 M), whereas the PdAu was comparatively less affected (∼50% activity decrease). All catalysts remained active at TDS levels as high as 100 »000 ppm. The majority of the hydroxylamine formed N2, however this reaction generated additional nitrite/nitrate anion byproducts with nitrogen selectivities ranging from 0.5% to 11.5%, depending on the catalyst identity and reaction salinity. To demonstrate one possible flow treatment process concept, we constructed and tested a recirculating trickle bed reactor that removed 28% phenol from simulated HFPW over 48 h. These results show the potential of oxidation catalysis as a treatment approach for produced water and other high-salinity industrial wastewaters.